amd64 front end: try to avoid a Memcheck false positive related to CPUID. n-i-bz.

amd64 front end: try to avoid a Memcheck false positive related to CPUID.  n-i-bz.

In the amd64 front end, CPUID is implemented by calling dirty helper.  The way
the side-effects for this call are declared can lead to false positives from
Memcheck.  This is a somewhat inelegant "fix", but it's the least-worst that
can be done without changing parameter-passing for the helper functions
involved.  A big in-line comment explains the problem and fix.